Long-term exposure to an invasive fungal pathogen decreases Eptesicus fuscus body mass with increasing latitude

Invasive pathogens threaten wildlife health and biodiversity. Physiological responses of species highly susceptible to pathogen infections following invasion are well described. However, the responses of less susceptible species (relative to highly susceptible species) are not well known. Latitudinal gradients, which can influence body condition via Bergmann's rule and/or reflect the time it takes for an introduced pathogen to spread geographically, add an additional layer for how mammalian species respond to pathogen exposure. Our goal was to understand how hosts less susceptible to pathogen infections respond to long-term pathogen exposure across a broad latitudinal gradient. We examined changes in body mass throughout pathogen exposure time across the eastern United States (latitude ranging 30.5 degrees N-44.8 degrees N) in Eptesicus fuscus , a bat species classified as less susceptible to infection (relative to highly susceptible species) by the invasive fungal pathogen that causes white-nose syndrome, Pseudogymnoascus destructans (Pd). Using 30 years of spring through fall adult capture records, we created linear mixed-effects models for female and male bats to determine how mass or mass variation changed across the eastern United States from pre -Pd invasion years through Pd invasion (0-1 years with Pd), epidemic (2-4 years with Pd), and established years (5+ years with Pd). By Pd establishment, all female and male bats decreased body mass with increasing latitude across a spatial threshold at 39.6 degrees N. Differences in bat mass north and south of the spatial threshold progressively increased over Pd exposure time-steps such that body mass was lower in northern latitudes compared to southern latitudes by Pd establishment. Results indicated that the progressive differences in E. fuscus body mass with latitude across the eastern United States are due to long-term pathogen exposure; however, other environmental and ecological pressures may contribute to decreases in E. fuscus body mass with latitude and long-term pathogen exposure. As pathogen introductions and emerging infectious diseases become more prevalent on the landscape, it is imperative that we understand how less susceptible species directly and indirectly respond to long-term pathogen exposure in order to maintain population health in surviving species.